For measuring biomagnetic signals, magnetometers such as SQUID sensors are usually used, which are sensitive just to dynamic phenomena. In this manner, the DC currents of an object that is immovable with respect to a set of sensors do not cause a measurement signal; and the only way to measure the DC currents is to move the object with respect to the set of sensors. In that case, a static magnetic field distribution produced by the DC currents in the co-ordinates of the object changes in the co-ordinates of the set of sensors as a function of time, thus causing a measurement signal that changes as a function of time.
The DC currents producing DC fields are not usually very interesting, but e.g. in the magnetoencephalographic i.e. MEG measurements there are situations in which it is desirable to perceive DC currents. Interesting DC currents are associated e.g. with epilepsy, migraine and REM phases of sleep.
In addition to the DC fields caused by physiological DC currents, DC fields are produced by all the immovable magnetised articles in the co-ordinates of an object. These can include tiny magnetic particles left in the skull by a drill used in a brain surgery, as well as magnetic impurities e.g. in the hair. As the object moves, magnetisations of this kind typically produce a very strong interference signal compared to a biomagnetic signal, the elimination and attenuation of which is necessary in order to perceive the physiological phenomenon being studied.
The problem is a typical one specifically in clinical measurements which measure patients who find it difficult to stay completely immovable during the measurement. In addition to the MEG measurements, the DC fields produced by DC currents can be of importance e.g. in the magnetocardiogpraphic i.e. MKG measurements to be made to study cardiac functions, in which magnetised particles produce a measurement signal e.g. as a result of respiratory movements.
To perceive physiological DC currents, a method has been used in which a testee is moved with respect to his or her geometry in a manner known se, e.g. periodically at a known frequency and amplitude with respect to the measuring instrument. One such method has been described e.g. in the publications “Measurement of near-DC biomagnetic fields of the head using a horizontal modulation of the body position”, Wuebbeler et al, Recent Advances in Biomagnetism, Sendai, pp. 369-372, 1999 and “Hyperventilation-induced human cerebral magnetic fields non-invasively monitored by multichannel ‘direct current’ magnetoencephalography”, Carbon et al, Neuroscience Letters, Vol. 287, pp. 227-230, 2000. In the method in question, the testee is lying on a bed that is movable with respect to the set of sensors so that the testee's head is supported to be immovable with respect to the bed. This must be done in order that the movement of the head can be assumed to correspond to a known movement of the bed. The bed is moved sinusoidally at the frequency of 0.4 Hz and at the amplitude of 75 mm, whereby the DC currents of the head are visible in the measurement signal at the modulation frequency of 0.4 Hz. The signals are demodulated and reconstructed in a manner enabling one- to easily study the DC signals.
The method described above relates to the measurement of interesting physiological DC currents using a magnetoencephalographic apparatus. In the method, the head's own movement is prevented and a movement that is necessary for perceiving DC signals is produced using a means, i.e. a bed. In that case, the magnetisation of the bed also produces a signal of the modulation frequency, which as being an interference signal, must be eliminated e.g. by moving the bed in a corresponding manner without the testee, and by measuring the DC signal due to this so as to be the reference.
Several problems and limitations are associated with the method described above. Especially patients in poor health may experience fastening their head unpleasant. Furthermore, the movement of the bed produces the aforementioned interference signal, the elimination of which, as well as the building of a mechanical movement system and the preparation of DC measurements require a lot of additional work compared to a conventional MEG measurement. Thus, the method is very susceptible to interference.
To eliminate the interference signals produced by “additional” DC fields of a movable testee that are associated with conventional MEG measurements, one has not presented any manner based on the DC property of interference sources. The methods of interference elimination do not take into account the movement of the testee; instead they just try to eliminate the interference signal caused by movement from the measurements using standard methods. This can be implemented e.g. using high-pass filtering, but the slow brain signals are lost at the same time.